The most politically effective celebrities of all time.

Francis X. Bushman is almost forgotten now, but a century ago he was one of the first silent film stars, a handsome, powerfully built leading man. Traveling by train one day to a benefit in Boston with Mary Pickford, another star of the silent screen often called “America’s Sweetheart,” Bushman found himself summoned to a meeting with William Howard Taft, who was also on board.

To Bushman’s astonishment, the president told the actor that he “envied” him. “He’d watched the demonstration at the station—Mary had had her clothes almost torn off her,” Bushman told an interviewer decades later, “and I looked like the wrath of God.” Taft, who was heading for a landslide defeat in 1912, saw in the demonstration all the support that he had lost. “All the people love you,” the president lamented, “and I can’t have even the love of half the people.”

The encounter between Bushman and Taft is a reminder that the fascination between Washington and Hollywood—which will be on glittering display again this weekend at the White House Correspondents’ Association Dinner—is a much older phenomenon than is often recognized. Al Jolson, then a Broadway star but soon The Jazz Singer in Hollywood’s first “talkie,” led delegations of entertainers into the Republican presidential campaigns of Warren Harding in 1920 and Calvin Coolidge in 1924. (“Keep cool with Coolidge,” Jolson warbled.) Louis B. Mayer, the formidable final M in MGM (Metro-Goldwyn-Mayer) was a close friend and adviser to Herbert Hoover, not usually considered among the more starstruck presidents. Franklin D. Roosevelt, who wasn’t starstruck either, bedazzled Hollywood; even John F. Kennedy and Bill Clinton couldn’t match the star power that assembled for FDR: immortals such as Frank Sinatra, Judy Garland, Humphrey Bogart, Orson Welles, Jimmy Cagney, and Groucho Marx.

The relationship between the capital of glamour and the capital of power has endured for decades, but it hasn’t remained static. From the 1920s to the 1960s, most stars (with such exceptions as Welles and Melvyn Douglas) saw themselves in supporting roles—they were more likely to wave from the stage than deliver a speech from it. As all institutions lost authority in the 1960s, that deference crumbled, and stars became increasingly willing to assert their personal views. “Who’s to say who’s an expert?” Paul Newman tellingly declared during the 1968 presidential campaign, when he invested his formidable fame in Eugene McCarthy’s primary challenge to President Johnson. “Just because I can sway more people than I have a right to, does that mean I’m not entitled to my opinions or to voice them?”

In the decades since, the star with a cause has become a mainstay of both political and cultural life. Rare is the political campaign or cause so obscure that it can’t attract even a single celebrity. Stars, in fact, now participate in politics so routinely that in their visits to Washington or campaign appearances most of them leave no more imprint than a breeze along the beach in Malibu. Not infrequently, celebrities have stumbled on political terrain, seeming self-righteous or shallow: think of Jane Fonda on the left or Anita Bryant on the right. Even as colossal a cultural force as Oprah Winfrey was seared by a scandal at a girls’ school she built in South Africa.

But over the years, some entertainers have made a lasting mark on politics and public affairs. In the special report that follows, National Journal offers its list of the 20 (22 if you include couples) most politically effective celebrities of all time. We defined celebrities as actual entertainers, not executives or producers. Our measure of effectiveness was this: Which stars made the greatest positive impact on their chosen causes or campaigns?

To answer that question, NJ interviewed dozens of staff members on Capitol Hill and asked the Republican and Democratic operatives whom it regularly consults on other issues for its Insiders Poll which contemporary stars have been the most effective advocates in Washington and the most valuable surrogates on the campaign trail. But the final list is inherently subjective. It represents the assessment of NJ’s editors and is intended not as a last word but as the invitation to start a conversation.

Like everything else in Hollywood, political activism has been in and out of vogue over the years. The past decade has produced a relatively high level of engagement, but activism of a distinct kind. Two important trends are driving contemporary activism among entertainers. One is a shift away from electoral campaigns toward direct philanthropic action. The second is a shift from domestic concerns toward international issues, particularly the intertwined challenges of poverty, health, and violence in Africa and other parts of the developing world. The cumulative effect is that the biggest stars today are less likely to be found campaigning for a candidate in Detroit than visiting a refugee camp in Darfur.